The invention relates generally to medical imaging. In particular, the invention relates to digital X-ray medical imaging systems having a flat-panel digital X-ray detector.
The bone mineral density (BMD) of a bone reflects the strength of the bone as represented by calcium content. It is defined as the integral mass of bone mineral per unit of projected area in grams per square centimeter. BMD is a useful tool for the diagnosis and treatment of several diseases and conditions, one of which is osteoporosis.
Osteoporosis is a disease of bone in which the BMD is reduced due to depletion of calcium and bone protein. Osteoporosis predisposes a person to fractures, which are often slow to heal and heal poorly. It is more common in older adults, particularly post-menopausal women; in patients on steroids; and in those who take steroidal drugs. Unchecked osteoporosis can lead to changes in posture, physical abnormality (particularly a condition known colloquially as “dowager's hump”), and decreased mobility. Treatment of osteoporosis includes ensuring that the patient's diet contains adequate calcium and other minerals needed to promote new bone growth, and for post-menopausal women, estrogen or combination hormone supplements.
Dual-energy X-ray absorptiometry (DXA or DEXA) is an increasingly important bone density measurement technology. In fact, osteoporosis is defined by the World Health Organization (WHO) as a BMD having a value 2.5 standard deviations below peak bone mass (in a 20-year-old sex-matched healthy person average) as measured by DXA. The fundamental principle behind DXA is the measurement of the transmission of X-rays with two different energy levels. By measuring how much X-ray energy is transmitted through the patient, the amount of X-ray energy that is absorbed in the patient can be determined. Soft tissues and bone absorb the two energy level X-rays to different degrees. As a result, the absorption of X-rays by the soft tissue may be distinguished from the absorption of X-rays by bone. The soft tissue image data may then be subtracted from the bone image data, leaving only the image data for bone. The BMD is then determined from the bone image data.
However, a BMD alone may not be sufficient for treatment. Evidence of spinal fractures is another important indicator of bone conditions. Determining whether a fracture is present is important both for treatment and for research purposes. A patient may display a reduced BMD, but a physician may be hesitant or unwilling to begin a particular treatment without a diagnosis of a fracture or a deformity. In a research setting, a diagnosis of fracture is important in studying the incidence and prevalence of osteoporosis in a population, as an entry criterion to a clinical study, or as a measure of efficacy with regard to a particular treatment. In fact, the European Foundation for Osteoporosis has published guidelines for clinical trials in osteoporosis which recommends a definition of osteoporosis as “a disorder where one or more fractures have arisen due to an increase in the fragility of bone.” In addition, they propose that studies of the efficacy of new drugs used in treatment of osteoporosis have fracture reduction as their clinical endpoint.
While the presence or absence of vertebral fracture is critical in the diagnosis of osteoporosis, diagnosis of vertebral fracture is often difficult. Over one-half of such fractures are asymptomatic, and in cases of minimal symptoms obvious fracture or deformity will often not be observed, particularly if there is no previous radiological record for comparison. Vertebral morphometry techniques promise to make the determination of vertebral fracture or deformation more objective. These approaches rely on certain indexes or normative values of vertebral body dimensions. In using vertebral morphometry to diagnose fractures, the clinician commonly employs analog radiological imaging techniques. In essence, an analog or digital X-ray image of the patient's vertebrae is taken, and printed onto a fixed media, such as an X-ray radiographic film print. The print is made to a specific scale relative to the patient, e.g., one-to-one, or a specifically reduced or expanded scale. Then the clinician manually measures the size of a vertebra by using a ruler and a straight edge and actually draws on the film to outline the vertebral body, and then measures with the ruler between criteria lines drawn onto the film itself.
There have been recent efforts to computerize this morphometric technique. These efforts still rely on first obtaining an analog X-ray image of the vertebra, digitizing the analog image and then manually selecting the points of measurement. Thus the clinician diagnosing or treating osteoporosis must, at a minimum, use two relatively expensive medical devices: a bone densitometer and an X-ray imaging device. Further, morphometric techniques which rely on analog or digital radiography are complicated by image magnification. The analog/digital radiographic image is typically 10-15% larger than life-size, and the magnification is variable depending on the location of the object relative to the plane of the radiograph. In particular, the front edge of the object, away from the radiographic plate is more magnified than the back edge toward the radiographic plate. The result is that bone edges perpendicular to the plane of the plate, which for morphological measurement should produce a sharp visual demarcation on the fan beam radiograph produce a blurred boundary. Distortions of the spine are particularly acute for cone beam exposures at the edges of the cone beam where the beam is most angled. For vertebral morphology the angulation obscures and distorts intervertebral spacing at the top and bottom of a field rendering morphological measurements imprecise. This impreciseness is exacerbated by the imposition of human error when it is left to the clinician to manually select the measurement point. In addition, variation will often necessarily exist between clinicians and between measurements by the same clinician at different times.
While bone densitometers are capable of generating images, the image quality of these present day bone densitometers is inferior to that of common analog or digital X-ray imaging systems. This is particularly true for scanning systems where resolution is intentionally limited to prevent the need for an overly long scanning time. Thus, the imaging capability of bone densitometers has not been relied on for diagnostic purposes and until the present invention, bone densitometry systems have not been used to determine bone morphology, or to analyze the relationships of bone structures. In addition, there is a need to perform the bone densitometry in the same radiology room using the similar acquisition conditions including the magnification and resolution etc. Unfortunately, a diagnostic X-ray image is also not quantitative due to the scatter present in image.
Therefore, a need exists for a technique that combines the beneficial aspects of a diagnostic X-ray image with the quantitative information provided by a BMD acquisition.